Field of the Invention
The present invention relates to the testing of signal levels within a measuring system and, more particularly, to built-in, self-testing circuitry for checking signal levels at system test points during normal system operation.
With the advent of present day advances in mircroprocessor and software technology, many measuring and analytical instruments such as spectrophotometers, liquid scintillation counters, light scattering photometers, and the like are incorporating processors of this nature for processing and measuring information derived by the measuring instrument. The instruments are typically designed to perform sophisticated analysis of clinical, biological, or other samples for laboratories, hospitals and the like. As the complexity of such instruments increases, it is evident that an instrument manufacturer must provide the requisite backup technical support to a user to advise the user regarding instrument operation and to diagnose and remedy malfunctions in instrument operation.
The cost of providing skilled technical support and repair personnel can be high, and for this reason it is desirable to derive an instrument design which enables a relatively unskilled operator or user to perform at least limited trouble-shooting on the instrument with minimal assistance from the instrument manufacturer. For example, the source of a malfunction can often be traced by simply checking signal levels at various test points strategically located within the circuitry of the system. If the user could test these signal levels, then the user might isolate the problem and be able to correct it. If assistance is needed, the user could provide the test results to the manufacturer by telephone and receive corrective instructions without the need for a service visit by the manufacturer. However, even if a service visit were required, providing the test results to the manufacturer ahead of time would enable the service representative to accurately diagnose the problem and bring the necessary components and equipment for repair.
Troubleshooting and instrument in the foregoing manner obviously requires a voltmeter or other signal measuring device. This presents a practical difficulty since many instrument users simply do not have, or cannot justify the expense of acquiring, a separate voltmeter for infrequent use in troubleshooting an instrument.
Because of this limited user capability, some instruments have been designed with a built-in voltmeter feature. With this approach, a plurality of leads are hard wired to preselected test points in the system circuitry. A multi-position switch, usually hand operated, is provided to connect respective leads in succession to a built-in voltmeter, and the voltmeter reading for each is displayed. While this approach does provide valuable diagnostic information, it exhibits several drawbacks which reduce its overall usefulness. First, when the test operations are being performed, the normal measuring operation of the system is disabled. For example, if the system normally measures a changing optical or electrical characteristic of a chemical reaction between sample materials, this measuring function is not conducted during the diagnostic test procedure. However, often a system malfunction is apparent only during an actual measuring operation. In such case it would be mandatory to test signal levels during the measuring operation to isolate at what point during a measurement and in what manner the malfunction occurs. This is not possible with the foregoing system.
Another disadvantage of the foregoing system resides in the limited number of test points which can be tested. Hard wiring the test leads between the pre-selected test points and the multi-position switch precludes an operator from checking any but the hard wired test points.